Glass ceramic material, method for producing the same and spark plug containing such a glass ceramic material

ABSTRACT

A glass ceramic which is especially suitable as a resistor ( 5 ) or a gas-tight glass ceramic solder ( 9 ) in a spark plug ( 1 ) is described. The glass ceramic here is a fused seal of a starting glass which is fused from a starting mixture containing SiO 2 , Al 2 O 3 , TiO 2  and CaO. The fused seal also has crystalline phases in at least some areas. In addition, a method is described for producing such a glass ceramic, the starting glass being processed in a first method step to form a starting material. This starting material is then heated for a first period of time in a second method step from a starting temperature, which is below the softening temperature (T g ) of the starting glass, to a fusion temperature, which is above the softening temperature (T g ) of the starting glass, and is kept at that temperature for a second period of time and finally is cooled again. Finally, a spark plug is described which has a terminal stud ( 3 ) and a center electrode ( 6 ) which are electrically connected across a resistor ( 5 ) that is formed in at least some areas by the glass ceramic proposed here.

[0001] The present invention relates to a glass ceramic, in particular for use as a resistor or as a gas-tight ceramic solder in a spark plug, a method of producing same and a spark plug having such a glass ceramic according to the definition of the species of the independent claims.

BACKGROUND INFORMATION

[0002] German Patent Application 196 51 454.1 describes a spark plug in which a terminal stud is connected to a center electrode across a resistor in the form of a fused glass seal arranged between the electrode and the terminal stud. In addition, it is already known that this resistor seal may be made of a glass material or a glass ceramic material which may be provided with a metal phase in the form of a network to increase the electric conductivity. This metal phase is achieved by a surface metallization of glass powder deposited in a currentless operation and then fused in the spark plug to form a resistor.

[0003] In addition, German Patent Application 196 23 989.3 also describes a spark plug in which the terminal stud is connected to the center electrode by a burn-off resistor. It is also provided there for a contact pin to be arranged between the burn-off resistor and the center electrode.

ADVANTAGES OF THE INVENTION

[0004] The glass ceramic according to the present invention has the advantage in comparison with the related art that it is suitable for use as a glass ceramic seal in a spark plug where the seal may have either a high or low resistance, depending on its composition. It is advantageously stable at temperatures up to more than 1000° C., so that it can be used in particular as a burn-off resistor in a spark plug, which is greatly preferred. Thus on the whole, the glass ceramic according to the present invention is also suitable for use in novel spark plugs having a platinum center electrode which is heated up to 950° C. in the area of the insulator base during operation. In particular, it can tolerate an operating temperature of 900° C. for more than 2000 hours.

[0005] The glass ceramic according to the present invention also has the advantage that it can be fused from a glass powder or a glass powder mixture at a process temperature of less than 950° C., especially between 850° C. and 950° C., without requiring the use of a protective gas. Furthermore, it has high voltage strength up to 20 kV/mm at room temperature or up to 10 kV/mm at 800° C., its thermal expansion coefficient being adapted at the same time to that of aluminum oxide (Al₂O₃) as the typical insulator material used in the case of spark plugs. In concrete terms, the thermal expansion coefficient typically amounts to approx. 6 ppm/K at 100° C. to 200° C. and approx. 9 ppm/K at 700° C. to 800° C. On the basis of these properties, the glass ceramic according to the present invention is therefore especially suitable for producing a glass ceramic seal as a burn-off resistor in a spark plug having an adjacent insulator made of dense, pure crystalline aluminum oxide.

[0006] The high temperature stability and high voltage strength are also achieved due to the fact that the glass ceramic according to the present invention has refractory phases, in particular the phases anorthite, wollastonite and titanite, at least partially or at least in some areas.

[0007] It is also advantageous that through controlled processing of the starting glass to a starting material, a glass ceramic can also be produced according to the present invention in the form of an electrically conducting glass ceramic solder which is used, for example, for contacting a resistor seal made of a glass ceramic having a different composition with a metal, such as a contact pin, a stud or a center electrode in a spark plug.

[0008] The glass ceramic according to the present invention in particular makes it possible to guarantee thermally stable and gas-tight contacting of the center electrode or the stud in a spark plug in an advantageous manner by varying its composition.

[0009] Advantageous refinements of the present invention are derived from the measures characterized in the subordinate claims.

[0010] With regard to the high voltage strength, thermal stability and adaptation of the thermal expansion coefficient to the surrounding insulator made of aluminum oxide, it is especially advantageous if the glass ceramic has a composition according to claim 4.

[0011] An especially advantageous embodiment of the glass ceramic with regard to the development of refractory phases through a controlled temperature treatment of the starting glass is obtained when the starting glass used to produce the glass ceramic has a composition according to claim 5.

[0012] In order to adjust the electric properties of the glass ceramic to be produced, there are advantageously two different possibilities that can be used as alternatives: first, the development of a metal phase which is embedded in the glass ceramic and is designed in particular in the form of a network, or on the other hand the development of a carbon phase which is also embedded in the glass ceramic and is designed in particular in the form of a network. The carbon phase here is preferably composed of pyrolyzed carbon.

[0013] To produce a glass ceramic [having] the desired crystalline phases, it is also advantageous if the fusion temperature of the starting material used is between 850° C. and 950° C. The heated starting material is then kept at this temperature for a first period of time, preferably amounting to 5 minutes to 15 minutes, whereupon the refractory phases mentioned are formed and the starting material is converted to a glass ceramic. A glass ceramic is understood in this connection to be a material which, unlike a glass, has crystalline phases in at least some areas.

[0014] To produce a glass ceramic having a low resistance, in particular for use as a glass ceramic solder, the glass powder used in the course of the method according to the present invention may be advantageously provided with a metallization composed of a metal with high temperature stability on the one hand or, on the other hand and alternatively, it may be provided with a compound of a glass powder, a binder and a carbon black powder.

DRAWINGS

[0015] The present invention is explained in greater detail on the basis of the drawings and in the following description. FIG. 1 shows as a first embodiment a section through a spark plug having a glass ceramic seal as a burn-off resistor. FIG. 2 shows an alternative embodiment to FIG. 1.

EMBODIMENTS

[0016] First, to implement a glass ceramic seal as a burn-off resistor or as a ceramic solder in a spark plug 1 in an essentially known manner, a starting glass is fused from a starting mixture. The starting mixture here is composed of 38 to 48 wt % SiO₂, 15 to 19 wt % Al₂O₃, 4.5 to 11 wt % TiO₂, 0 to 1.5 wt % Na₂O, 0 to 1.5 wt % K₂O and 23 to 30 wt % CaO. In addition, Li₂O may also be added to the starting mixture in an amount of up to 1.5 wt %.

[0017] Preferably the starting mixture is composed of 43 to 48 wt % SiO₂, 16.5 to 18 wt % Al₂O₃, 6 to 10.5 wt % TiO₂, 0.3 to 1.2 wt % Na₂O, 0.3 to 1.2 wt % K₂O and 24.5 to 28.5 wt % CaO. The best results are obtained with a starting mixture which is composed of 45 wt % SiO₂, 17 wt % Al₂O₃, 9 wt % TiO₂, 0.5 wt % Na₂O, 0.5 wt % K₂O and 28 wt % CaO.

[0018] To produce a burn-off resistor in the form of a glass ceramic resistor seal which is connected by a glass ceramic solder to adjacent metallic parts of the spark plug, the starting mixture described above is first fused to form a starting glass which is then split into two batches. Then the first batch of this starting glass is ground to form a glass powder having a mean particle size of 150 μm to 250 μm.

[0019] The glass powder prepared in this way is then subsequently used as the actual fused high-resistance resistor seal in the form of a burn-off resistor in a spark plug.

[0020] The second batch of the starting glass is subsequently used to produce a glass ceramic solder for joining this burn-off resistor to adjacent metal parts in the spark plug.

[0021] To produce this glass ceramic solder in the form of a low-resistance glass ceramic, there are two alternative possibilities for its production.

[0022] The first embodiment provides for the starting glass to be first ground to form a glass powder having a mean particle size of less than 250 μm and then to be provided with a surface metallization by currentless metallization by the method described in German Patent 196 51 454.1. This surface metallization is a metallization with a metal having a high-temperature stability, such as platinum, palladium, nickel, tungsten or an alloy of these metals. Palladium is preferred. The thickness of the surface metallization is usually between 0.5 nm and 10 nm, especially between 2 nm and 5 nm.

[0023] As an alternative to the surface metallization of the glass powder ground from the starting glass as explained above, however, it may be first processed to form a starting material in a first method step. To do so, a first portion of the starting glass is ground to form a first glass powder having a mean particle size of 150 μm to 250 μm, and a second portion of the starting glass is ground to form a second glass powder having a mean particle size of less than 100 μm, in particular from 5 μm to 70 μm. These two glass powders having different particle sizes are then mixed with a carbon black powder and an organic binder. The carbon black powder has a mean particle size of 200 nm to 2 μm, especially from 400 nm to 600 nm. The organic binder is a mixture of carboxymethylcellulose and dextrin, using water as the solvent. In addition, zirconium dioxide powder and mullite powder are also added to the starting material in a preferred embodiment, each having a mean particle size of less than 100 μm.

[0024] Specifically, the starting glass is processed in a first method step to form a starting material containing an amount of 40 wt % to 58 wt % of the first glass powder, an amount of 3 wt % to 13 wt % of the second glass powder, an amount of 0.9 wt % to 2.5 wt % of the carbon black powder, an amount of 10 wt % to 37 wt % of the zirconium dioxide powder, an amount of 8 wt % to 13 wt % of the mullite powder and an amount of 0.6 wt % to 4 wt % of the organic binder. The values given in wt % above are based on the solvent-free starting material to which the solvent is subsequently added, namely water in the example described here. The amount of solvent in the starting material is 12 to 40 vol %, in particular 22 to 37 vol %.

[0025] After the starting glass has been prepared in the form of a glass powder and on the other hand, a surface-metallized glass powder or alternatively, a processed starting material of a glass powder mixture has been prepared, these materials are then used in the production of a spark plug 1. This will be explained with the help of FIG. 1.

[0026]FIG. 1 shows a spark plug 1, which is known in principle from German Patent 196 51 454.1 and has an insulator 2 made, for example, of pure, crystalline and gas-tight aluminum oxide, a metallic terminal stud 3, a housing 4, a resistor 5 in the form of a glass ceramic resistor seal, a center electrode 6 which is made of platinum, for example, or is coated with platinum, and a ground electrode 7. Terminal stud 3 also has a lower end 8.

[0027] Between lower end 8 of terminal stud 3 and resistor 5, a gas-tight glass ceramic solder 9 is also provided, connecting resistor 5 and terminal stud 3 together so they are gas-tight. In addition, a gas-tight glass ceramic solder 9 is also provided between center electrode 6 and resistor 5, connecting resistor 5 to center electrode 6 so they are gas-tight.

[0028] Spark plug 1 also has an upper end 10, a thread 11 and a terminal nut 12 for connecting an ignition line. In addition, a leakage current barrier 13 and a polygonal arrangement 16 are provided, with the help of which spark plug 1 can be screwed into an engine block. A thread 17 is provided for this purpose. Center electrode 6 and ground electrode 7 are separated by a spacer 18. Finally, FIG. 1 also shows that insulator 2 protrudes to a great extent, shielding center electrode 6 from ground electrode 7 in some areas. For this purpose, insulator 2 has an insulator base tip 19, so that a spark gap can be established only between the tip of center electrode 6 and ground electrode 7. Spark plug 1 also has an end face 20 which limits the extent to which spark plug 1 can be screwed into an engine block.

[0029] To produce the spark plug according to FIG. 1, insulator 2 is first provided with center electrode 6 in a known manner. Then either the glass powder which has been provided with a surface metallization is packed into the cavity in insulator 2 as glass ceramic solder 9, or the prepared starting material composed of the glass powder mixture, carbon black, organic binder and optionally other ingredients is packed into the cavity. Then to produce high-resistance resistor 5, the starting glass which has been ground to form a glass powder is packed into insulator 3. Finally, again either the surface-metallized glass powder or the starting material which has been prepared in the manner described above is added on top of this ground starting glass. Finally, terminal stud 3 is placed on top of this sequence of layers.

[0030] There then follows the second method step for production of a glass ceramic in which the different materials introduced into the insulator are heated for a first period of time from a starting temperature, which is below the softening temperature T_(g) of the starting glass, to a fusion temperature, which is above the softening temperature T_(g) of the starting glass, to form resistor 5 or gas-tight glass ceramic solder 9 together with the spark plug and inserted stud 3. Spark plug 1 is then kept at this temperature for a second period of time and finally is cooled again.

[0031] The starting temperature in the heat treatment described above is typically between 10° C. and 40° C., but is especially at room temperature. The fusion temperature is 850° C. to 950° C. The first period of time during which the spark plug is heated together with the added starting materials is between 5 and 15 minutes, especially between 8 and 10 minutes. The second period of time during which the spark plug is kept at the fusion temperature thus reached is between 5 and 25 minutes, especially between 9 and 15 minutes. In the course of this heat treatment, i.e., during the fusion operation, crystallization also occurs at least partially and/or at least in some areas in the starting glass. This results in formation of, among other things, the refractory phases anorthite (CaO.Al₂O₃.2SiO₂), wollastonite (CaO.2SiO₂) and titanite (CaO.TiO₂.SiO₂).

[0032] Thus on the whole, after this heat treatment, a seal is formed between center electrode 6 and stud 3 in the form of a glass ceramic which has a typical resistance of more than 1 kΩ in the area of resistor 5 and forms a high-resistance seal as a burn-off resistor. In addition, two areas have been formed with a gas-tight glass ceramic solder 9 connecting resistor 5 to center electrode 6 and stud 3. Depending on the composition selected, gas-tight glass ceramic solder 9 may be a glass ceramic including a metal phase embedded in it, in particular designed in the form of a network, in at least some areas, so that this metal phase essentially carries the electric conductivity of the glass ceramic solder. On the other hand, the glass ceramic solder 9 may also be in the form of a glass ceramic having a carbon phase embedded in it in at least some areas, in particular designed in the form of a network. This carbon phase is formed by pyrolysis from the organic binder and/or carbon black powder added to the prepared starting material.

[0033] Glass transition temperature T_(g) of the starting glass is 753° C. in the example illustrated here, but it may also be between 670° C. and 780° C., depending on the composition of the starting glass. The dilatometric softening temperature E_(g) of the starting glass thus produced is 786° C. in the example described here, but it may also vary between approx. 720° C. and approx. 820° C., depending on the composition of the starting glass. The thermal expansion coefficient a of the starting glass is 8.3×10⁻⁶/K at a temperature of 100° C. to 500° C., but it may also vary between 6.7×10⁶/K and 8.8×10⁶/K within the scope of the limits of the composition as defined above.

[0034]FIG. 2 shows an alternative embodiment of spark plug 1 in comparison with FIG. 1. The spark plug according to FIG. 2 is known in principle from German Patent Application 196 23 989.3. It differs from the spark plug according to FIG. 1 especially through the shape of ground electrode 7 and center electrode 6. In addition, in the case of spark plug 1 according to FIG. 2, resistor 5 is set back in comparison with FIG. 1, with a terminal stud 21 also being provided between resistor 5 and center electrode 6. Production of the spark plug according to FIG. 2 is completely analogous to that in FIG. 1 with regard to the design of resistor 5 and gas-tight glass ceramic solder 9. According to FIG. 2, resistor 5 is not connected on its lower side to center electrode 6, however, and instead is connected in a gas-tight manner to terminal stud 21 by glass ceramic solder 9. 

What is claimed is:
 1. A glass ceramic, in particular for use as a resistor (5) or as a gas-tight glass ceramic solder (9) in a spark plug (1) having a starting glass which is fused from a starting mixture containing SiO₂, Al₂O₃, TiO₂ and CaO, wherein the glass ceramic is a fused seal of the starting glass having crystalline phases in at least some areas.
 2. The glass ceramic according to claim 1, wherein the starting mixture contains 38 wt % to 48 wt % SiO₂, 15 wt % to 19 wt % Al₂O₃, 4.5 wt % to 10 wt % TiO₂, 0 wt % to 1.5 wt % Na₂O, 0 wt % to 1.5 wt % K₂O and 23 wt % to 30 wt % CaO.
 3. The glass ceramic according to claim 1 or 2, wherein lithium oxide is added to the starting mixture in an amount of up to 1.5 wt %.
 4. The glass ceramic according to claim 1, wherein the starting mixture contains 43 wt % to 48 wt % SiO₂, 16.5 wt % to 18 wt % Al₂O₃, 6 wt % to 10.5 wt % TiO₂, 0.3 wt % to 1.2 wt % Na₂O, 0.3 wt % to 1.2 wt % K₂O and 24.5 wt % to 28.5 wt % CaO.
 5. The glass ceramic according to claim 1, wherein the starting mixture is composed of 45 wt % SiO₂, 17 wt % Al₂O₃, 9 wt % TiO₂, 0.5 wt % Na₂O, 0.5 wt % K₂O and 28 wt % CaO.
 6. The glass ceramic according to at least one of the preceding claims, wherein the glass ceramic contains the refractory phases anorthite, wollastonite and titanite.
 7. The glass ceramic [according to] at least one of the preceding claims, wherein the glass ceramic includes in at least some areas a metal phase which is embedded in it and is designed in particular in the form of a network.
 8. The glass ceramic according to at least one of the preceding claims, wherein the glass ceramic includes a carbon phase which is embedded in it and is designed in particular in the form of a network, at least in some areas.
 9. A method of producing a glass ceramic according to at least one of the preceding claims, wherein the starting glass is processed in a first method step to form a starting material, this starting material is then heated for a first period of time in a second method step from a starting temperature, which is below the softening temperature (T_(g)) of the starting glass, to a fusion temperature, which is above the softening temperature (T_(g)) of the starting glass, and is kept at that temperature for a second period of time and finally is cooled again.
 10. The method according to claim 9, wherein the starting temperature is between 10° C. and 40° C. and the fusion temperature is between 850° C. and 950° C.
 11. The method according to claim 9 or 10, wherein the first period of time is between 5 and 15 minutes, in particular between 8 and 10 minutes, and the second period of time is between 5 and 25 minutes, in particular between 9 and 15 minutes.
 12. The method according to claim 9, wherein processing of the starting glass in the first method step includes milling to form a glass powder, in particular having a mean particle size of less than 250 μm.
 13. The method according to claim 12, wherein the glass powder is provided at least partially with a surface metallization in the first method step.
 14. The method according to claims 12 or 13, wherein the glass powder is provided with a metallization with a metal that is stable at high temperatures, such as platinum, palladium, nickel, tungsten or alloys thereof having a thickness of 0.5 nm to 10 nm.
 15. The method according to claim 9, wherein processing of the starting glass to form the starting material in the first method step includes milling a first portion of the starting glass to form a first glass powder having a mean particle size of 150 μm to 250 μm and milling a second portion of the starting glass to form a second glass powder having a mean particle size of less than 100 μm, in particular from 5 μm to 70 μm, as well as mixing this glass powder with a carbon black powder and an organic binder.
 16. The method according to claim 15, wherein the carbon black powder having a mean particle size of 200 nm to 2 μm, in particular from 400 nm to 600 nm, is used.
 17. The method according to claim 15, wherein a mixture of carboxymethylcellulose and dextrin with water as the solvent is used as the organic binder.
 18. The method according to at least one of claims 15 through 17, wherein zirconium dioxide in particular as a powder having a mean particle size of less than 100 μm is added when processing the starting powder to form the starting material.
 19. The method according to at least one of claims 15 through 18, wherein mullite is added when processing the starting powder to form the starting material.
 20. The method according to at least one of claims 15 through 19, wherein the starting glass is processed in the first method step to form a starting material containing an amount of 40 wt % to 58 wt % of the first glass powder, an amount of 3 wt % to 13 wt % of the second glass powder, an amount of 0.9 wt % to 2.5 wt % carbon black powder, an amount of 10 wt % to 37 wt % zirconium dioxide, an amount of 8 wt % to 13 wt % mullite and an amount of 0.6 wt % to 4 wt % binder, the amounts in wt % being based on the solvent-free starting material.
 21. The method according to at least one of claims 15 through 20, wherein the solvent is added to the starting material in an amount of 12 to 40 vol %, especially 22 vol % to 37 vol %.
 22. A spark plug having a terminal stud (3) and a center electrode (6) which are electrically connected across a resistor (5), wherein the resistor (5) is made in at least some areas of a glass ceramic according to at least one of claims 1 through
 8. 23. The spark plug according to claim 22, wherein a contact pin (21) is arranged between the resistor (5) and the center electrode (6).
 24. The spark plug according to claim 22, wherein the resistor (5) connected to the terminal stud (3) and/or the center electrode (6) by a glass ceramic solder made of a glass ceramic according to at least one of claims 1 through
 8. 25. The spark plug according to claim 23, wherein the resistor (5) is connected to the terminal stud (3) and/or the contact pin (21) by a glass ceramic solder made of a glass ceramic according to at least one of claims 1 through
 8. 